The present invention relates to an EDM method and apparatus for cutting a workpiece by way of an electrode tool in the form of an electrically conductive wire, ribbon or tape, all generally referred to as wire electrodes, the wire electrode being displaced relative to the workpiece along a path corresponding generally to the longitudinal axis of the wire, while being simultaneously fed along a cutting path at an angle to such longitudinal axis, so as to effectuate a cut in the electrode workpiece as a result of voltage pulses applied across the electrodes triggering electrical current discharges to pass through the machining zone formed between the two electrodes. The wire electrode is fed and stretched between a pair of support and guide members as a result of a predetermined force of traction being exerted on the wire electrode in the general direction of its longitudinal axis.
In the course of cutting the electrode workpiece, the wire electrode is subjected to forces which are applied transversely. Some of those forces tend to repel the wire electrode from the workpiece, as, for example, the reaction forces exerted on the wire electrode by the miniature explosions occurring in the machining fluid when each electrical discharge passes through the fluid in the machining zone, while other forces cause the wire electrode to be attracted toward the workpiece, as, for example, electrostatic and electromagnetic forces resulting respectively from the voltage across the electrodes and the machining current flow. When the resultant force of all the component forces of repulsion and attraction is applied in a direction other than the longitudinal axis of the wire electrode, the wire is deflected transversely relative to its longitudinal axis of motion, as defined by its support and guide members, until an equilibrium is achieved under the influence of an additional component force directed in a transverse direction resulting, for example, from a variation of the intensity of the traction force applied to the wire. Unwanted deflections of the wire result in machining errors, more particularly when the cutting path through the workpiece is along a curve of small radius.
Methods are known which permit to decrease such machining errors, and which consist either in acting upon the forces of repulsion acting on the wire electrode (U.S. Pat. No. 4,081,652, issued Mar. 28, 1978, and assigned to the same assignee as the present application), or on the forces of traction (U.S. Pat. No. 4,104,502, issued Aug. 1, 1978, and assigned to A. G. fur industrielle Elektronik). The first of such methods presents, however, the inconvenience of slowing down the machining speed, and the second does not permit to adjust the amplitude of the correcting factors to be applied upon the wire for maintaining the wire correctly aligned with its support and guide members.
Another method, disclosed in application Ser. No. 843,431, filed Oct. 19, 1977, now abandoned and assigned to the same assignee as the present application, consists in correcting the preprogrammed cutting path in the workpiece by taking into consideration the permanent deviation of the wire. However, the continuous occurrence of the deviation of the wire results in a machining error in the course of cutting a path forming a sharp angle, as a result of the curvilinear shape taken by the wire in the machining zone.